Direct-sequence (DS) spread-spectrum modulation is commonly used in wireless communication systems based on code-division multiple-access (CDMA), where each information symbol is represented by a number of “chips.” Representing one symbol by many chips gives rise to “spreading,” as the latter typically requires more bandwidth to transmit. The sequence of chips is referred to as the spreading code. At the receiver, the received signal is despread using a despreading code, which is typically the conjugate of the spreading code.
Interference and noise are the main signal impairments affecting receiver performance in DS-CDMA systems. Interference, in particular, is a combination of many components, including: self-interference, such as inter-symbol interference (ISI); multiple access interference, i.e., interference due to non-zero code cross-correlation; interference from other cells in the downlink; or interference from other users in the uplink. These impairment components must be suppressed at the receiver in order to achieve good throughput of the system.
One promising approach to suppressing impairment is linear equalization. Linear equalization can be performed either before despreading (referred to as chip-level equalization) or after despreading (referred to as symbol-level equalization). In symbol-level equalization, the received chip-level data is despread at multiple delays, and then the multiple signal images are weightedly combined. Chip-level equalization reverses the order of these operations. The received chip data is first weightedly combined using a linear filter and then despread at a single delay. Under most circumstances, symbol-level and chip-level equalization provide equivalent performance
Where the combining weights are computed based on signal characteristics, e.g. a signal or impairment covariance matrix, those weights may be estimated from either the chip-level or despread versions of the signal. In some scenarios, the combining weights computed from symbol-level signal characteristics are simply scaled versions of the combining weights computed from chip-level signal characteristics. Combining weight computation may therefore be conceptually separated from combining weight application, since, if desired, the weights could be computed at the chip-level, scaled, and then applied in a symbol-level equalization process.
Some contexts, however, threaten this relationship between chip-level and symbol-level equalization weight computation and otherwise threaten the ability of combining weights actually computed at the chip-level to provide the same level of equalization performance as that of combining weights actually computed at the symbol-level. In one such context, spreading codes associated with different impairment components are correlated with the spreading code of a desired component to different degrees, meaning that the despreading process affects different impairment components differently.